1. Field of the Invention
This invention relates generally to optical communication technology and, in particular, to an array of free space optical switches comprising electro-optic material that rely on total internal reflection to switch an optical beam.
2. Description of the Related Art
Demands for transmitting signals optically is growing at a rapid pace. Optically transmitted signals are typically in digital format, and may be carried through some form of a waveguide such as an optical fiber.
A network such as a telecommunication network, whether optical or not, typically requires a system of switches to be able to route the signals to proper destinations. As use herein, switching refers to directing the signal from one path to another path as desired. One method of switching optical signals is to first convert a plurality of optical signals into a plurality of electrical signals using an array of photo-sensitive detectors such as PIN-diodes. The switching can be performed on these resulting electrical signals, and the outgoing electrical signals can be converted back to optical signals using an array of devices such as laser diodes. Such a method requires a substantial infrastructure to provide proper optical-electrical and electrical-optical conversions.
One method of switching optical signals utilizes a total internal reflection (TIR) switch comprising two portions, at least one of which comprises electro-optic material whose refractive index can be altered by application of an electric field. The two portions are positioned adjacent to each other so as to define a boundary between them. By altering the refractive index of at least one of the portions, a sharp gradient in the refractive index can be formed at the boundary. Light input into the switch and incident on the boundary at an appropriate angle can be total internally reflected down one path. In a separate mode, the electric field has a value such that TIR does not occur, and the light input into the switch is substantially transmitted through the boundary and proceeds along another path.
Discussions of TIR switches in prior art, show the TIR switches in conjunction with waveguides formed on a substrate such as lithium niobate. Waveguides are used to interconnect TIR switches in optical switching arrays. See, e.g., U.S. Pat. No. 5,732,177 issued to Deacon et al. However, formation of the waveguides on the substrate, while well known in the art, requires numerous processing steps using expensive equipment. Fabrication of waveguides, thus, adds a level of complexity, and requires specialized production equipment.
Hence, a method of fabricating a TIR switch array in a more simple and economical manner is needed.
In one aspect of the invention, a switch array comprises a substrate having a surface, a plurality of TIR switches, and at least one input collimator. Each of these switches has a TIR surface and is mounted such that the TIR surface extends substantially orthogonal to the surface of the substrate. The at least one input collimator is mounted to receive an input beam from an optical fiber and transmit a collimated input beam towards at least one of the TIR surfaces. The switch array may further comprise at least one coupling element positioned to receive the collimated input beam and couple it to an optical fiber as an output beam. The array further comprises a free space region between adjacent TIR switches. The free space region comprises material that is substantially optically transmissive to the collimated input beam. This free space region is also substantially devoid of boundaries that limit the beam size of the collimated input beam traveling between the switches so as to provide for free space propagation of the collimated input beam. By having free-space regions and not waveguides interconnecting the switches, fabrication of the switch array can be simplified.
The free space region is preferably dimensioned to substantially exceed the beam size. The collimated input beam has a maximum beam diameter between about 30 xcexcm and 300 xcexcm, and the free space region is dimensioned to exceed this beam diameter.
The free space region may comprise an open region, or alternatively, the free region may comprises a solid material. The substantially optically transmissive solid material in free space region may be selected from the group consisting of glass, quartz, silicon dioxide, sapphire, brookite and rutile. In one embodiment, the substrate comprises a material selected from the group consisting of glass, quartz, silicon, sapphire, brookite and rutile. In one embodiment, the switches are imbedded in the substrate such that the free space region comprises the substrate material.
The switch array may further comprise a free space region between the input collimator and at least one of the TIR switches. This free space region is substantially devoid of boundaries that limit the beam size of the collimated input beam travelling to the TIR switches so as to provide for free space propagation of the collimated input beam. The free space region between the collimator and the TIR switches is dimensioned to substantially exceed the beam size of the collimated input beam, and in one embodiment, the dimension exceeds 30 xcexcm.
The switch array may further comprise a free space region between at least one of the switches and at least one of the optical coupling elements. This free space region is also substantially devoid of boundaries that limit the collimated input beam travelling between the switch and the optical coupling element so as to provide for free space propagation of the collimated input beam. The collimated input beam passing through the free space region between the switch and the optical coupling element has a maximum beam size and the free space region is dimensioned to substantially exceed the beam size.
The TIR switches preferably include a portion comprising electro-optic material sandwiched between two electrodes. The electo-optic portion forms a boundary from which the collimated beam is totally internally reflected when the switch is in one state. The electro-optic material is embedded in the substrate and the boundary may be formed between the electro-optic material and a portion of the substrate. Alternatively, the boundary is formed between the electro-optic material and another substantially optically transmissive material formed on the substrate, both of which are imbedded in the substrate. In one embodiment, the electro-optic material and the another substantially optically transmissive material are surrounded by electrically insulating material. The substrate may comprise silicon and the electrically insulating material may comprise silicon dioxide. In one embodiment, the boundary is formed between the electro-optic material and an open region.
In another aspect of the invention, an apparatus comprises an array of spaced-apart TIR switches and a collimator which receives light from an optical fiber and transmits a collimated beam through the array. The space between the switches is less than the Rayleigh range of the collimator. Preferably, the Rayleigh range is between about 100 micrometers (xcexcm) and 5 centimeters (cm), and more preferably between about 100 micrometers (mm) and 5 millimeters (mm). In one embodiment, the collimator has a diameter between about 125 xcexcm and 500 xcexcm. The apparatus may additionally comprise a coupling element which receives the collimated beam and couples it to an optical fiber as an output beam.
In yet another aspect of the invention, a method comprises providing a switch array comprised of TIR switches and transmitting an unguided collimated beam through a plurality of the switches in the switch array. Preferably, the substantially unguided collimated beam diverges such that the diameter of the beam increase by no more than a factor of about {square root over (2)} after being transmitted through the switches.
In still another aspect of the invention, a method of manufacturing an array of TIR switches comprises providing a slab comprised of a first optically transmissive material. A first plurality of substantially parallel channels is formed in the slab and the first plurality of channels is filled with a second optically transmissive material. At least one of the transmissive materials is electro-optically active, one of the transmissive materials is either electro-optically inactive or substantially less electro-optically active than the other. A second plurality of substantially parallel channels is also formed in the slab. The second plurality of channels at an angle relative to the first plurality. In addition, a third plurality of substantially parallel channels is formed in the slab. This third plurality of channels is at an angle relative to second plurality.
The first optically transmissive material may comprise electro-optic material and the second optically transmissive material may comprises non-electro-optic material. In another embodiment, the first optically transmissive material comprises non-electro-optic material and the second optically transmissive material comprises electro-optic material. The method may include depositing a substantially optically transmissive material on the array of TIR switches, and this material may be selected from the group consisting essentially of silicon dioxide, glass, sapphire, rutile, brookite and quartz.
In one embodiment, a fourth and a fifth plurality of substantially parallel channels are formed in the slab. The fourth and fifth pluralities of channels are also formed at an angle relative to second plurality. The channels may be formed in the slab by sawing.
The method may further comprise depositing conductive material to form electrodes on and under at least one electro-optic material.
In yet another aspect of the invention, a method of manufacturing an array of TIR switches includes providing a slab comprised of a optically transmissive electro-optically active material and forming a first plurality of substantially parallel channels in the slab. A second plurality of substantially parallel channels are also formed in the slab, the second plurality of substantially parallel channels being formed at an angle relative to the first plurality. A third plurality of substantially parallel channels are also formed in the slab at an angle relative to second plurality.